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This invention relates to quadrature amplitude modulation systems in general and particularly to receiving and demodulating compound quadrature modulated signals which are modulated onto subcarrier frequencies spaced from a central carrier frequency. In a quadrature modulation system, designed for radio transmission and reception, it is desirable to transmit a pilot carrier in order to facilitate reception and decoding of the transmitted information. This pilot signal is used for both amplitude and phase correction purposes. In a prior known approach, a pilot carrier has been added as a side frequency to quadrature amplitude modulated signals. Such a side signal can be subjected to the selective fading that occurs on a radio path resulting in problems in attempting to use the pilot carrier signal. Additionally, such a side signal may fall outside of the passband of a receiver.
Where digital information is to be received in a synchronous manner, channel fading and noise can prevent the maintenance of bit sync. In such a system, it is desirable to provide a bit sync signal with the synchronous digital information to be used in decoding the transmitted digital information.. Like the pilot carrier, the bit sync signal should be provided as close to the center of the transmitted frequency spectrum as possible.